In <f8ral8lndkanp4gd70pu1fmblq4g912o74@4ax.com>, on 03/29/2013 at 06:07 AM, quasi <quasi@null.set> said:

>There are certain concepts for which sets are inadequate and classes>come to the rescue. For example, we need the class concept if we>want to define an equivalence on the collection of all groups, since>that collection is not a set.

There's no such set in ZFC, but there are set theories in which itexists aqnd is a set.